If nothing else, today's Democratic primary debate in Nevada may be the first time some American voters realise that someone other than Hillary Clinton is running for the party's nomination for president.

The frontrunner is a known quantity, which isn't necessarily a good thing.

Mrs Clinton's campaign has struggled since revelations surfaced earlier this year that she used a private email server while she was secretary of state, a move that is now the subject of an FBI investigation.

For better or worse, the four men she will share the stage with in Las Vegas don't come with the same degree of name recognition.

Her closest rival, Bernie Sanders, is a self-declared democratic socialist who is calling for a people-power revolution to take on Wall Street and rebuild the country's "disappearing middle class".

Well known among his largely millennial supporters — wooed by his plans to offer free education and reduce inequality — the 74-year-old could try to use the debate to demonstrate that he can attract broad-based support for his views, which are seen by some as too far to the left, even for liberals.

The other three contenders — Martin O'Malley, Jim Webb and Lincoln Chafee — have received little media coverage to date and are barely registering on national polls. In many ways on debate night they have little to lose and a lot to gain.

Former Maryland governor O'Malley was originally pegged as Mrs Clinton's natural rival on her left because of his early advocacy for same-sex marriage and his liberal views on immigration, but that space that has been unexpectedly filled by Senator Sanders.

Mr O'Malley has been begging for the Democratic leadership to schedule more debates to force Mrs Clinton to share the spotlight. Having been so hungry for the national spotlight, a poor performance at the debate could have wider repercussions for his campaign.

Mr Webb, a former US senator from Virginia, was the secretary of the navy in the Regan administration and is known for advocating prison reform, opposing the Iraq war and attacking the nuclear deal with Iran.

Mr Chafee, a former governor of Rhode Island, is probably the most unlikely contender to be vying for the democratic nomination.

The former Republican was the only senator to vote against his party's invasion of Iraq in 2002. He left the GOP in 2007, and if he is known for anything these days, it's his desire for America to convert to the metric system.

The non-Clintons on the Vegas stage have little to lose in the debate and plenty to gain — rising from obscurity is a worthy ambition on its own. A strong performance could put a candidate on a rival's list of possible running mates, another formidable goal in a race that is currently made up of four Davids and one Goliath.

Senator Sanders has avoided attacking the frontrunner to date, but his rivals might not be so shy.

The email scandal makes Mrs Clinton an easy target, as does her reversal on policies like the Trans Pacific Partnership (TPP) trade deal which she supported while working for the Obama administration but has now denounced.

The former senator and secretary of state is expected to use the debate to show her human side and dismiss criticism that she has been a late adopter of progressive policy positions.

There is not nearly as much excitement around the Democratic debate compared to the two already held by the Republican Party.

The fiery first Republican showdown, featuring bombastic businessman Donald Trump, whose reputation for flinging insults and trading blows with opponents is well known, attracted a record TV audience.

The battle on the Vegas Strip probably won't be nearly as entertaining. Instead, it is shaping up to be less a clash of personalities and more a clash of ideas.